Effects of liquid diet intake on nerve growth in salivary glands of growing rats.

Abstract

The growth of parotid glands is inhibited by liquid diet intake during growing period, while that of submandibular glands is not affected. This study examined how liquid diet intake affects nerve growth in the parotid and submandibular glands of growing rats, because nerves are closely involved in the maintenance of salivary gland structure. Male Wistar rats were weaned at 21 days of age. Then, rats were fed a pellet diet and a liquid diet in the control group and experimental group, respectively. At 0, 2, 4, or 8 weeks, they were euthanised by isoflurane overdose, and parotid and submandibular glands were removed. The frozen sections were made and immuno-stained with anti-protein gene product 9.5 (PGP 9.5) antibody (general nerve marker), anti-tyrosine hydroxylase (TH) antibody (sympathetic nerve marker), or anti-neuronal nitric oxide synthase (nNOS) antibody (parasympathetic nerve marker). In control parotid glands, scattered punctate or short linear patterns of PGP 9.5-positive sites were observed at week 0. After 2 weeks, PGP 9.5-positive sites, some of which were arranged in long linear patterns, increased in number. There were some TH-positive sites at week 0. After 2 weeks, there were increasing numbers of TH-positive sites, often in long linear patterns. At week 0, there were very few nNOS-positive sites, and nNOS immunoreactivity increased over time. After week 4, they demonstrated linear patterns. In the experimental parotid glands, there were fewer PGP 9.5- and nNOS-positive sites than in control parotid glands at each time point, although TH immunoreactivity was similar between two groups at each time point. In control submandibular glands, few punctate exhibited PGP 9.5-positive site were observed at week 0. At week 4, PGP 9.5 immunoreaction increased and showed linear patterns. TH-positive sites demonstrated punctate or short linear patterns at week 0, and thereafter TH immunoreactivity increased and were arranged in long linear patterns. There were few nNOS-positive sites at week 0, and they gradually increased after week 4. The immunoreactivities of all antibodies in the experimental submandibular glands were similar to those in the control at each time point. Parasympathetic nerve growth in rat parotid glands was inhibited by liquid diet intake during the growth period, while liquid diet intake did not affect parasympathetic nerve growth nor sympathetic nerve growth in rat submandibular glands.